Negative electrode active material, negative electrode and battery

ABSTRACT

Provided is a negative electrode active material that can improve the discharge capacity per volume and/or charge-discharge cycle characteristics. The negative electrode active material according to the present embodiment contains an alloy phase and ceramics. The alloy phase undergoes thermoelastic diffusionless transformation when releasing or occluding metal ions. The ceramics is dispersed in the metal phase. The content of ceramics in the alloy phase is more than 0 to 50 mass % with respect to the total mass of the alloy phase and the ceramics.

TECHNICAL FIELD

The present invention relates to an electrode active material, and moreparticularly to a negative electrode active material.

BACKGROUND ART

Recently, small electronic appliances such as home video cameras, notePCs, and smart phones have become widespread, and attaining highercapacity and longer service life of batteries has become a technicalproblem.

Given that hybrid vehicles, plug-in hybrid vehicles, and electricvehicles will be further spread, size reduction of batteries is also atechnical problem.

At present, graphite-based negative electrode active materials areutilized for lithium ion batteries. However, graphite-based negativeelectrode active materials have technical problem as described above.

Accordingly, alloy-based negative electrode active materials have gainedattention, which have higher capacity than those of the graphite-basednegative electrode active materials. As an alloy-based negativeelectrode active material, silicon (Si)-based negative electrode activematerials and tin (Sn)-based negative electrode active materials areknown. To realize a lithium ion battery having a smaller size and alonger life, various studies have been conducted on the above describedalloy-based negative electrode active materials.

However, an alloy-based negative electrode active material repeatedlyundergoes large expansion and contraction in volume at the time ofcharging/discharging. For that reason, the capacity of the alloy-basednegative electrode active material is prone to deteriorate. For example,a volume expansion/contraction ratio of graphite associated withcharging is about 12%. In contrast, the volume expansion/contractionratio of Si single substance or Sn single substance associated withcharging is about 400%. For this reason, if a negative electrode plateof Si single substance or Sn single substance is repeatedly subjected tocharging and discharging, significant expansion and contraction occur,thereby causing cracking in negative electrode mixture which is appliedon the current collector of the negative electrode plate. Consequently,the capacity of the negative electrode plate sharply decreases. This ischiefly caused by the fact that some of the active substances are freeddue to volume expansion/contraction and thereby the negative electrodeplate loses electron conductivity.

US2008/0233479A (Patent Literature 1) proposes a method for solving theabove described problem of an alloy-based negative electrode activematerial. To be specific, the negative electrode material of PatentLiterature 1 includes a Ti—Ni superelastic alloy, and Si particlesformed in the superelastic alloy. Patent Literature 1 describes that alarge expansion/contraction change of Si particle which occur followingocclusion and release of lithium ions can be suppressed by asuperelastic alloy.

CITATION LIST Patent Literature

-   Patent Literature 1: US2008/0233479A

However, it is dubious that the technique disclosed in Patent Literature1 sufficiently improves the charge-discharge cycle characteristics ofthe secondary battery. Most of all, it may be highly difficult toactually produce the negative electrode active material proposed byPatent Literature 1.

SUMMARY OF INVENTION

It is an objective of the present invention to provide a negativeelectrode active material which can improve the discharge capacity pervolume and/or charge-discharge cycle characteristics thereof.

The negative electrode active material according to the presentembodiment contains an alloy phase and ceramics. The alloy phaseundergoes thermoelastic diffusionless transformation when releasing oroccluding metal ions. The ceramics is dispersed in the alloy phase. Thecontent of ceramics in the alloy phase is more than 0 to 50 mass % withrespect to the total mass of the alloy phase and the ceramics.

The negative electrode active material of the present embodiment canimprove the discharge capacity per volume and/or the charge-dischargecycle characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of D0₃ structure.

FIG. 2A is a schematic diagram of D0₃ structure of the matrix phase ofthe alloy phase of the present embodiment.

FIG. 2B is a schematic diagram of 2H structure of γ1′ phase which is akind of martensite phase.

FIG. 2C is a schematic diagram of a crystal plane to explainthermoelastic diffusionless transformation from D0₃ structure to 2Hstructure.

FIG. 2D is a schematic diagram of another crystal plane different fromthat of FIG. 2C.

FIG. 2E is a schematic diagram of another crystal plane different fromthose of FIGS. 2C and 2D.

DESCRIPTION OF EMBODIMENTS

Hereinafter, with reference to the drawings, embodiments of the presentinvention will be described in detail. Like parts or corresponding partsin the drawings are given a like reference symbol and descriptionthereof will not be repeated.

The negative electrode active material according to the presentembodiment contains an alloy phase and ceramics. The alloy phaseundergoes thermoelastic diffusionless transformation when releasing oroccluding metal ions. The ceramics is dispersed in the metal phase. Thecontent of ceramics in the alloy phase is more than 0 to 50 mass % withrespect to the total mass of the alloy phase and the ceramics.

A “negative electrode active material” referred herein is preferably anegative electrode active material for nonaqueous electrolyte secondarybatteries. A “thermoelastic diffusionless transformation” referredherein is so-called thermoelastic martensitic transformation. A “metalion” refers to, for example, a lithium ion, magnesium ion, sodium ion,and the like. A preferable metal ion is lithium ion.

This negative electrode active material may contain other phasesdifferent from the above described alloy phases. The other phasesinclude, for example, a silicon (Si) phase, a tin (Sn) phase, otheralloy phases (alloy phases which do not undergo thermoelasticdiffusionless transformation) excepting the above described alloyphases, and the like.

Preferably, the above described alloy phases are main components (mainphases) of the negative electrode active material. “Main component”refers to a component which occupies not less than 50% by volume. Thealloy phase may contain impurities to the extent that the spirit of thepresent invention is unimpaired. However, the impurities are containedpreferably as little as possible.

A negative electrode formed of a negative electrode active material ofthe present embodiment has a higher volumetric discharge capacity(discharge capacity per volume) than that of a negative electrode madeof graphite, when used in a nonaqueous electrolyte secondary battery.Further, a nonaqueous electrolyte secondary battery using a negativeelectrode containing a negative electrode active material of the presentembodiment has a higher capacity retention ratio than one using aconventional alloy-based negative electrode. Therefore, the negativeelectrode active material can sufficiently improve the charge-dischargecycle characteristics of the nonaqueous electrolyte secondary battery.

A possible reason why the capacity retention ratio is high is thatstrain due to expansion/contraction that occurs at the time ofcharging/discharging is relaxed by thermoelastic diffusionlesstransformation. Moreover, it is considered that containing a specificamount of ceramics dispersed in the alloy phase contributes to a highcapacity retention ratio. Detailed description thereof will be givenbelow.

The ceramics preferably contains at least one or more kinds selectedfrom the group consisting of Al₂O₃, FeSi, SiC, Si₃N₄, TiC, TiB₂, Y₂O₃,ZrB₂, HfB₂, ZrO₂, ZnO, WC, W₂C, CrB₂, BN, and CeO₂.

The alloy phase may be of any one of the following types 1 to 4.

The alloy phase of type 1 undergoes thermoelastic diffusionlesstransformation when occluding metal ions, and undergoes reversetransformation when releasing metal ions. In this case, the alloy phaseis a mother phase in a normal state.

The alloy phase of type 2 undergoes reverse transformation whenoccluding metal ions, and undergoes thermoelastic diffusionlesstransformation when releasing metal ions. In this case, the alloy phaseis a martensite phase in a normal state.

The alloy phase of type 3 undergoes supplemental deformation (slipdeformation or twin deformation) when occluding metal ions, and returnsto the original martensite phase when releasing metal ions. In thiscase, the alloy phase is a martensite phase in a normal state.

The alloy phase of type 4 transforms from a martensite phase to anothermartensite phase when occluding metal ions, and returns to the originalmartensite phase when releasing metal ions. In this case, the alloyphase is a martensite phase in a normal state.

In the case of the alloy phase of type 1, preferably, the crystalstructure of the alloy phase after thermoelastic diffusionlesstransformation is either of 2H, 3R, 6R, 9R, 18R, M2H, M3R, M6R, M9R, andM18R in the Ramsdell notation, and the crystal structure of the alloyphase after reverse transformation is D0₃ in the Strukturberichtnotation. More preferably, the crystal structure of the alloy phaseafter thermoelastic diffusionless transformation is the above described2H, and the crystal structure of the alloy phase after reversetransformation is the above described D0₃.

In the case of the alloy phase of type 1, preferably, the alloy phasecontains Cu and Sn, and also contains the above described 2H structureafter thermoelastic diffusionless transformation, and the abovedescribed D0₃ structure after reverse transformation.

The above described alloy phase may contain one or more selected fromthe group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B, and C,and Sn, with balance being Cu and impurities.

The above described alloy phase may contain one or more selected fromthe group consisting of δ phase of F-Cell structure, ε phase of 2Hstructure, η′ phase of monoclinic crystal, and a phase having D0₃structure, each including site deficiency.

All of these δ phase, ε phase, η′ phase, and phase having D0₃ structure,each including site deficiency form a storage site and a diffusion siteof metal ions (Li ions, etc.) in the alloy phase. Thereby, thevolumetric discharge capacity and the cycle characteristics of thenegative electrode active material are further improved.

In the above described negative electrode active material, a volumeexpansion ratio or volume contraction ratio of a unit cell of the abovedescribed alloy phase before and after the phase transformation ispreferably not more than 20%, and more preferably not more than 10%. Thevolume expansion ratio of unit cell is defined by the following Formula(1), and the volume contraction ratio of unit cell is defined by thefollowing Formula (2).(Volume expansion ratio of unit cell)=[(volume of unit cell when metalions are occluded)−(volume of unit cell when metal ions arereleased)]/(volume of unit cell when metal ions are released)×100  (1)(Volume contraction ratio of unit cell)=[(volume of unit cell when metalions are occluded)−(volume of unit cell when metal ions arereleased)]/(volume of unit cell when metal ions are occluded)×100  (2)

The volume of unit cell at the time of releasing, which corresponds to acrystal lattice range of unit cell at the time of occluding, issubstituted into “volume of unit cell when metal ions are released” inFormulas (1) and (2).

The above described negative electrode active material can be used asactive material for making up an electrode, particularly electrode of anonaqueous electrolyte secondary battery. An example of the nonaqueouselectrolyte secondary battery is a lithium ion secondary battery.

Hereinafter, negative electrode active materials according to thepresent embodiment will be described in detail.

<Negative Electrode Active Material>

The negative electrode active material according to the presentembodiment contains an alloy phase and ceramics. The ceramics isdispersed in the alloy phase. The content of ceramics in the alloy phaseis more than 0 to 50 mass % with respect to the total mass of the alloyphase and the ceramics.

[Alloy Phase]

The alloy phase undergoes thermoelastic diffusionless transformationwhen releasing metal ions represented by Li ions, or occluding the metalions, as described above. The thermoelastic diffusionless transformationis also called as thermoelastic martensitic transformation. Hereinafter,in the present description, the thermoelastic martensitic transformationis simply referred to as “M transformation” and the martensite phase as“M phase”. An alloy phase that undergoes M transformation when occludingor releasing metal ions is also referred to as a “specific alloy phase”.

The specific alloy phase is dominantly made up of at least one of Mphase and a matrix phase. The specific alloy phase repeatsocclusion/release of metal ions at the time of charging/discharging.Then, the specific alloy phase undergoes M transformation, reversetransformation, supplemental deformation, etc. in response to occlusionand release of metal ions. These transformation behaviors mitigatestrain which is caused by expansion and contraction of the alloy phasewhen occluding and releasing metal ions.

The specific alloy phase may be of any one of the above described types1 to 4. Preferably, the specific alloy phase is of type 1. That is, thespecific alloy phase preferably undergoes M transformation whenoccluding metal ions, and undergoes reverse transformation whenreleasing metal ions.

The crystal structure of the specific alloy phase is not specificallylimited. If the alloy phase is of type 1, and the crystal structure ofthe specific alloy phase (that is, a matrix phase) after reversetransformation is β₁ phase (D0₃ structure), the crystal structure of thespecific alloy phase (that is, M phase) after M transformation is, forexample, β₁′ phase (M18R₁ structure of monoclinic crystal or 18R₁structure of orthorhombic crystal), γ₁′ phase (M2H structure ofmonoclinic crystal or 2H structure of orthorhombic crystal), β₁″ phase(M18R₂ structure of monoclinic crystal or 18R₂ structure of orthorhombiccrystal), α₁′ phase (M6R structure of monoclinic crystal or 6R structureof orthorhombic crystal), and the like.

If the crystal structure of the matrix phase of the specific alloy phaseis β₂ phase (B2 structure), the crystal structure of M phase of thespecific alloy phase is, for example, β₂′ phase (M9R structure ofmonoclinic crystal or 9R structure of orthorhombic crystal), γ₂′ phase(M2H structure of monoclinic crystal or 2H structure of orthorhombiccrystal), and α₂′ phase (M3R structure of monoclinic crystal or 3Rstructure of orthorhombic crystal).

If the matrix phase of the alloy phase has a face-centered cubiclattice, the crystal structure of M phase of the alloy phase has, forexample, a face-centered tetragonal lattice, and a body-centeredtetragonal lattice.

Such symbols as the above described 2H, 3R, 6R, 9R, 18R, M2H, M3R, M6R,M9R, and M18R are used as the method of denoting crystal structures of alayered construction according to Ramsdell's classification. The symbolsH and R mean that respective symmetries in the direction perpendicularto the lamination plane are hexagonal symmetry and rhombohedralsymmetry. If there is no M appended at the beginning, it means that thecrystal structure is an orthorhombic crystal. If there is M appended atthe beginning, it means that the crystal structure is a monocliniccrystal. Even if same classification symbols are used, there are casesin which distinction is made by the difference in the order of thelayers. For example, since β₁′ phase and β₁″ phase, which are two kindsof M phase, have a different layered construction, there are cases inwhich they are distinguished by being denoted as 18R₁ and 18R₂, or M18R₁and M18R₂ etc., respectively.

In General, M transformation and reverse transformation in normal shapememory effects and pseudoelastic effects often involve volumecontraction or volume expansion. When the alloy phase of a negativeelectrode active material relating to the present embodimentelectrochemically releases or occludes metal ions (for example, lithiumions), it is considered that the crystal structure often changes inconsistent with the phenomena of volume contraction or volume expansionin the direction of respective transformation.

However, the negative electrode active material according to the presentembodiment will not be particularly limited by such restriction. When Mtransformation or reverse transformation occurs following occlusion andrelease of metal ions in the specific alloy phase, there may begenerated other crystal structures than the crystal structure thatappears at the time of ordinary shape memory effects and pseudoelasticeffects.

When the specific alloy phase is of type 3, the specific alloy phaseundergoes slip deformation or twin deformation following occlusion orrelease of metal ions. In slip deformation, since dislocation isintroduced as the lattice defect, reversible deformation is difficult.Therefore, when the specific alloy phase is of type 3, it is preferablethat twin deformation dominantly occurs.

[Chemical Composition of Alloy Phase]

The chemical composition of the above described specific alloy phasewill not be particularly limited provided that the crystal structure atthe time of M transformation and reverse transformation contains theabove described crystal structures.

When the specific alloy phase is of type 1, the chemical composition ofthe specific alloy phase contains, for example, Cu (copper) and Sn(tin).

When the specific alloy phase is of type 1, preferably, the crystalstructure of the specific alloy phase after reverse transformationcaused by discharge of metal ions is D0₃ structure shown in FIG. 1, andthe crystal structure of the specific alloy phase after M transformationcaused by occlusion of metal ions is 2H structure.

Preferably, the chemical composition of a specific alloy phase containsSn, with the balance being Cu and impurities. More preferably, thespecific alloy phase contains 10 to 20 at % or 21 to 27 at % of Sn, withthe balance being Cu and impurities, wherein the negative electrodeactive material contains 2H structure after M transformation, and D0₃structure after reverse transformation. A more preferable Sn content inthe alloy phase is 13 to 16 at %, 18.5 to 20 at %, or 21 to 27 at %.

The chemical composition of a specific alloy phase may contain one ormore selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni,Zn, Al, Si, B, and C, and Sn, with the balance being Cu and impurities.

Preferably, the chemical composition of the specific alloy phase in thiscase contains: Sn: 10 to 35 at %, and one or more selected from thegroup consisting of Ti: 9.0 at % or less, V: 49.0 at % or less, Cr: 49.0at % or less, Mn: 9.0 at % or less, Fe: 49.0 at % or less, Co: 49.0 at %or less, Ni: 9.0 at % or less, Zn: 29.0 at % or less, Al: 49.0 at % orless, Si: 49.0 at % or less, B: 5.0 at % or less, and C: 5.0 at % orless, with the balance being Cu and impurities. The above described Ti,V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B and C are optional elements.

A preferable upper limit of Ti content is 9.0 at % as described above.The upper limit of Ti content is more preferably 6.0 at %, and furtherpreferably 5.0 at %. A lower limit of Ti content is preferably 0.1 at %,more preferably 0.5 at %, and further preferably at 1.0 at %.

A preferable upper limit of V content is 49.0 at % as described above.The upper limit of V content is more preferably 30.0 at %, furtherpreferably 15.0 at %, and furthermore preferably 10.0 at %. A lowerlimit of V content is preferably 0.1 at %, more preferably 0.5 at %, andfurther preferably at 1.0 at %.

A preferable upper limit of Cr content is 49.0 at % as described above.The upper limit of Cr content is more preferably 30.0 at %, furtherpreferably 15.0 at %, and furthermore preferably 10.0 at %. A lowerlimit of Cr content is preferably 0.1 at %, more preferably 0.5 at %,and further preferably at 1.0 at %.

A preferable upper limit of Mn content is 9.0 at % as described above.The upper limit of Mn content is more preferably 6.0 at %, and furtherpreferably 5.0 at %. A lower limit of Mn content is preferably 0.1 at %,more preferably 0.5 at %, and further preferably at 1.0 at %.

A preferable upper limit of Fe content is 49.0 at % as described above.The upper limit of Fe content is more preferably 30.0 at %, furtherpreferably 15.0 at %, and furthermore preferably 10.0 at %. A lowerlimit of Fe content is preferably 0.1 at %, more preferably 0.5 at %,and further preferably at 1.0 at %.

A preferable upper limit of Co content is 49.0 at % as described above.The upper limit of Co content is more preferably 30.0 at %, furtherpreferably 15.0 at %, and furthermore preferably 10.0 at %. A lowerlimit of Co content is preferably 0.1 at %, more preferably 0.5 at %,and further preferably at 1.0 at %.

A preferable upper limit of Ni content is 9.0 at % as described above.The upper limit of Ni content is more preferably 5.0 at %, and furtherpreferably 2.0 at %. A lower limit of Ni content is preferably 0.1 at %,more preferably 0.5 at %, and further preferably at 1.0 at %.

A preferable upper limit of Zn content is 29.0 at % as described above.The upper limit of Zn content is more preferably 27.0 at %, and furtherpreferably 25.0 at %. A lower limit of Zn content is preferably 0.1 at%, more preferably 0.5 at %, and further preferably at 1.0 at %.

A preferable upper limit of Al content is 49.0 at % as described above.The upper limit of Al content is more preferably 30.0 at %, furtherpreferably 15.0 at %, and furthermore preferably 10.0 at %. A lowerlimit of Al content is preferably 0.1%, more preferably 0.5 at %, andfurther preferably at 1.0 at %.

A preferable upper limit of Si content is 49.0 at % as described above.The upper limit of Si content is more preferably 30.0 at %, furtherpreferably 15.0 at %, and furthermore preferably 10.0 at %. A lowerlimit of Si content is preferably 0.1 at %, more preferably 0.5 at %,and further preferably at 1.0 at %.

A preferable upper limit of B content is 5.0 at %. The lower limit of Bcontent is preferably 0.01 at %, more preferably 0.1 at %, furtherpreferably 0.5 at %, and furthermore preferably 1.0 at %.

A preferable upper limit of C content is 5.0 at %. The lower limit of Ccontent is preferably 0.01 at %, more preferably 0.1 at %, furtherpreferably 0.5 at %, and furthermore preferably 1.0 at %.

Preferably, the specific alloy phase contains one or more selected fromthe group consisting of δ phase of F-Cell structure containing sitedeficiency, ε phase of 2H structure containing site deficiency, η′ phaseof monoclinic crystal containing site deficiency, and a phase having D0₃structure containing site deficiency. Hereinafter, these δ phase, εphase, η′ phase, and phase having D0₃ structure, each containing sitedeficiency is also referred to as “site deficient phase”. Here, “sitedeficiency” means a state of a crystal structure in which occupancyfactor is less than 1 in a specific atomic site.

These site deficient phases include a plurality of site deficiencies inthe crystal structure. These site deficiencies function as a storagesite or a diffusion site of metal ions (such as Li ions). Therefore, ifa specific alloy phase contains an alloy phase which becomes 2Hstructure after M transformation and becomes D0₃ structure after reversetransformation, and at least one phase among the above described sitedeficient phases, the volumetric discharge capacity and the cyclecharacteristics of the negative electrode active material are furtherimproved.

The chemical composition of a specific alloy phase may further contain aGroup 2 element and/or rare earth metal (REM) for the purpose ofincreasing discharge capacity. The Group 2 elements include, forexample, magnesium (Mg) calcium (Ca) and the like. REMs include, forexample, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd)and the like.

If a specific alloy phase contains a Group 2 element and/or REM, thenegative electrode active material becomes brittle. Therefore, in theproduction process of the electrode, a bulk material or an ingot made ofthe negative electrode active material is easy to be pulverized, makingit easy to produce an electrode.

The negative electrode active material may consist of the abovedescribed alloy phase and the ceramics dispersed in the alloy phase, ormay contain another active substance phase which is metal-ion activebesides the above described specific alloy phase and the ceramics.Another active material phase includes, for example, a tin (Sn) phase, asilicon (Si) phase, an aluminum (Al) phase, a Co—Sn alloy phase, aCu₆Sn₅ compound phase (η′ phase or η phase) and the like.

[Volume Expansion Ratio and Volume Contraction Ratio of Specific AlloyPhase]

When the above described specific alloy phase undergoes M transformationor reverse transformation following occlusion and release of metal ions,preferable volume expansion/contraction ratio of unit cell of thespecific alloy phase is not more than 20%. In this case, it is possibleto sufficiently relax the strain due to a volume change which occursfollowing occlusion and release of metal ions. The volumeexpansion/contraction ratio of unit cell of the specific alloy phase ismore preferably not more than 10%, and further preferably not more than5%.

The volume expansion/contraction ratio of the specific alloy phase canbe measured by an in-situ X-ray diffraction during charging/discharging.To be specific, an electrode plate of negative electrode activematerial, a separator, a counter electrode lithium, and electrolyticsolution are placed and sealed in a dedicated charge/discharge cellincluding a window made of beryllium which transmits X-ray, within aglove box in pure argon gas atmosphere in which moisture is controlledsuch that due point is not more than −80° C. Then, this charge/dischargecell is mounted onto the X-ray diffraction apparatus. After mounting, anX-ray diffraction profile of the specific alloy phase is obtained ineach of an initially charged state and an initially discharged state inthe course of charging and discharging. From this X-ray diffractionprofile, a lattice constant of the specific alloy phase is found. Fromthe lattice constant, it is possible to calculate the volume changeratio in consideration of crystal lattice correspondence of the specificalloy phase.

When the shape of X-ray diffraction profile changes due to full width athalf maximum etc. in the charge-discharge cycling process, analysis isperformed after repeating charging and discharging 5 to 20 times asneeded. Then, an average value of volume change ratio is found from aplurality of X-ray diffraction profiles having high reliability.

[Analysis Method of Crystal Structure of Alloy Phase Contained byNegative Electrode Active Material]

(1) The crystal structure of the phase (including an alloy phase)contained in the negative electrode active material can be analyzed byRietveld method based on the X-ray diffraction profile obtained by usingan X-ray diffraction apparatus. To be specific, the crystal structure isanalyzed by the following method.

For a negative electrode active material before use for a negativeelectrode, X-ray diffraction measurement is performed on the negativeelectrode active material to obtain measured data of X-ray diffractionprofile. Based on the obtained X-ray diffraction profile (measureddata), the configuration of phases in the negative electrode activematerial is analyzed by Rietveld method. For the analysis by Rietveldmethod, either of “RIETAN2000” (program name) or “RIETAN-FP” (programname) which are general-purpose analysis software is used.

(2) The crystal structure of a negative electrode active material in anegative electrode before charging in a battery is determined by thesame method as that in (1). To be specific, the battery, which is in anuncharged state, is disassembled within the glove box in argonatmosphere, and the negative electrode is taken out from the battery.The negative electrode taken out is enclosed with Myler foil.Thereafter, the perimeter of the Myler foil is sealed by athermocompression bonding machine. Then, the negative electrode sealedby the Myler foil is taken out of the glove box.

Next, a measurement sample is fabricated by bonding the negativeelectrode to a reflection-free sample plate (a plate of a silicon singlecrystal which is cut out such that a specific crystal plane is inparallel with the measurement plane) with hair spray. The measurementsample is mounted onto the X-ray diffraction apparatus and X-raydiffraction measurement of the measurement sample is performed to obtainan X-ray diffraction profile. Based on the obtained X-ray diffractionprofile, the crystal structure of the negative electrode active materialin the negative electrode is determined by the Rietveld method.

(3) Crystal structures of the negative electrode active material in thenegative electrode after charging one to multiple times and afterdischarging one to multiple times are determined by the same method asthat in (2).

To be specific, the battery is fully charged in a charging/dischargingtest apparatus. The fully charged battery is disassembled in the glovebox, and a measurement sample is fabricated by a method similar to thatof (2). The measurement sample is mounted onto the X-ray diffractionapparatus and X-ray diffraction measurement is performed.

Moreover, the battery is fully discharged, and the fully dischargedbattery is disassembled in the glove box and a measurement sample isfabricated by a method similar to that of (2) to perform X-raydiffraction measurement.

[Ceramics]

The negative electrode active material according to embodiments of thepresent invention contains ceramics dispersed in a specific alloy phase.Containing ceramics will improve the capacity retention ratio, therebyimproving the charge-discharge cycle characteristics. This is consideredto be due to the following mechanism. For example, when charging anddischarging are repeated multiple times such as not less than 100cycles, cracking may occur in the specific alloy phase. The presence ofceramics dispersed in the alloy phase can suppress such cracking frompropagating. Moreover, since ceramics does not relate to thecharge-discharge reaction with metal ions such as lithium ion, it doesnot exhibit volume expansion/contraction at the time of charging anddischarging. Therefore, it is considered that using the negativeelectrode active material relating to embodiments of the presentinvention improves the capacity retention ratio. It is desirable thatthe ceramics be uniformly dispersed.

The content of ceramics in the specific alloy phase is more than 0 tonot more than 50 mass % with respect to the total mass of the specificalloy phase and the ceramics. The reason why to make the content to benot more than 50 mass % is to ensure a sufficient discharge capacity.The upper limit is more preferably not more than 10 mass %, and furtherpreferably not more than 5 mass %. To sufficiently achieve the effect ofsuppressing crack propagation, the lower limit is more preferably notless than 0.01 mass %, and further preferably not less than 0.1 mass %.

Moreover, when a mechanical method such as mechanical grinding andmechanical alloying is used in the process of dispersing ceramics,strain is introduced into the specific alloy phase in the dispersionprocess. In this case, the diffusion/storage site of metal ionincreases, thereby increasing electric capacity as the negativeelectrode active substance. However, the effect of capacity increase isrestricted by the balance with the blending ratio of ceramics which iselectrochemically inactive. As a matter of course, when the blendingratio of ceramics is high, its effect will become limited.

The average grain size of the ceramics contained and dispersed in thespecific alloy phase is not particularly limited. Considering the effectof suppressing crack propagation, the average grain size of the ceramicsis preferably 1 nm to 500 nm.

Preferably, the ceramics to be dispersed in the specific alloy phase isfor example one or more kinds selected from the group consisting ofAl₂O₃, FeSi, SiC, Si₃N₄, TiC, TiB₂, Y₂O₃, ZrB₂, HfB₂, ZrO₂, ZnO, WC,W₂C, CrB₂, BN, and CeO₂. The reason why these ceramics are suitable foruse in the negative electrode active material of the present embodimentis that its fine dispersion effect into the alloy is high. Particularlypreferable ceramics is one or more kinds selected from the groupconsisting of Al₂O₃, FeSi, SiC, Si₃N₄, TiC, Y₂O₃, WC, and CeO₂.

<Production Method of Negative Electrode Active Material>

A negative electrode active material containing the above describedspecific alloy phase and a production method of a negative electrode anda battery using the negative electrode active material will bedescribed.

A melt of the negative electrode active material containing the specificalloy phase (without containing ceramics) is produced. For example,molten metal having the above described chemical composition isproduced. The molten metal is produced by melting starting material byan ordinary melting method such as arc melting or resistance heatingmelting. Next, an ingot (bulk alloy) is produced by an ingot castingmethod by using the molten metal. By the above described processes, analloy material as a material before ceramics is contained and dispersed(hereinafter, described simply as an “alloy material”) is produced.

The alloy material is produced, preferably by subjecting the moltenmetal to rapid solidification. This method is called a rapidsolidification method. Examples of the rapid solidification methodinclude a strip casing method, a melt-spin method, a gas atomizationmethod, a melt spinning method, a water atomization method, an oilatomization method, and the like.

When the alloy material is powdered, the bulk alloy (ingot) obtained bymelting is (1) cut, (2) coarsely crushed by a hammer mill etc., or (3)finely pulverized mechanically by a ball mill, an attritor, a disc mill,a jet mill, a pin mill, and the like to be adjusted into a necessarygrain size. When the bulk alloy has ductility and ordinary pulverizationis difficult, the bulk alloy may be subjected to cutting andpulverization by a grinder disc, which is embedded with diamond abrasiveparticles, and the like. When M phase due to stress induction is formedin these pulverization processes, the formation ratio thereof isadjusted as needed by appropriately combining the alloy design, heattreatment, and pulverization conditions thereof. When powder generatedby an atomization method can be used as melted or as heat treated, theremay be cases where no pulverization process is particularly needed.Moreover, when melted material is obtained by a strip casting method andcrushing thereof is difficult due to its ductility, the melted materialis adjusted to have a predetermined size by being subjected tomechanical cutting such as shearing. Moreover, in such a case, themelted material may be heat treated in a necessary stage, to adjust theratio between M phase and a matrix phase, and the like.

When the alloy material is heat treated to adjust the constitutionratio, etc. of the specific alloy phase, the alloy material may berapidly cooled after being retained at a predetermined temperature for apredetermined time period in inert atmosphere as needed. In thisoccasion, the cooling rate may be adjusted by selecting a quenchingmedium such as water, salt water with ice, oil, and liquid nitrogenaccording to the size of the alloy material, and setting the quenchingmedium to a predetermined temperature.

Next, ceramics is dispersed in the above described alloy material(dispersion process). To disperse ceramics, a mechanical method is mosteffective. For example, the alloy material and the ceramic particles arecharged into an instrument such as an attritor or a vibration ball mill,and the like, and the ceramics is dispersed in the alloy materialthrough mechanical grinding effect or mechanical alloying effect.Preferable average grain size of the ceramic particles to be used is 1nm to 500 nm as described above considering the effect of introductionof strain in the dispersion process. Performing mechanical alloying withthe elements constituting the alloy material and the ceramics as thestarting materials may be able to further enhance the dispersion effect.Through the above described processes, a negative electrode activematerial is produced.

<Production Method of Negative Electrode>

A negative electrode using a negative electrode active material relatingto an embodiment of the present invention can be produced by a methodwell known to those skilled in the art.

For example, a binder such as polyvinylidene fluoride (PVDF), polymethylmethacrylate (PMMA), polytetrafluoroethylene (PTFE), andstyrene-butadiene rubber (SBR) is admixed to powder of a negativeelectrode active material of an embodiment of the present invention, andfurther carbon material powder such as natural graphite, artificialgraphite, and acetylene black is admixed thereto to impart sufficientconductivity to the negative electrode. After being dissolved by addinga solvent such as N-methylpyrrolidone (NMP), dimethylformamide (DMF) andwater, the binder is stirred well using a homogenizer and glass beads ifnecessary, and formed into a slurry. This slurry is applied on an activesubstance support member such as a rolled copper foil and anelectrodeposited copper foil and is dried. Thereafter, the dried productis subjected to pressing. Through the above described processes, anegative electrode plate is produced.

The binder to be admixed is preferably about 5 to 10 mass % with respectto the mass of the entire slurry including the solvent from theviewpoints of mechanical strength and battery characteristics of thenegative electrode. The support member is not limited to a copper foil.The support member may be, for example, a foil of other metals such asstainless steel and nickel, a net-like sheet punching plate, a meshbraided with a metal element wire and the like.

The particle size of the powder of negative electrode active materialaffects the thickness and density of electrode, that is, the capacity ofelectrode. The thickness of electrode is preferably as thin as possible.This is because a smaller thickness of electrode can increase the totalsurface area of the negative electrode active material included in abattery. Therefore, an average particle size of the powder of negativeelectrode active material is preferably not more than 100 μm. As theaverage particle size of the powder of negative electrode activematerial decreases, the reaction area of the powder increases, therebyresulting in excellent rate characteristics. However, when the averageparticle size of the powder of negative electrode active material is toosmall, the properties and condition of the surface of the powder changedue to oxidation etc. so that it becomes difficult for lithium ions toenter into the powder. In such a case, the rate characteristics and theefficiency of charging/discharging may decline over time. Therefore, theaverage particle size of the powder of negative electrode activematerial is preferably 0.1 to 100 μm, and more preferably 1 to 50 μm.

<Production Method of Battery>

A nonaqueous electrolyte secondary battery according to the presentembodiment includes a negative electrode, a positive electrode, aseparator, and an electrolytic solution or electrolyte as describedabove. The shape of the battery may be a cylindrical type, a squareshape as well as a coin type and a sheet type. The battery of thepresent embodiment may be a battery utilizing a solid electrolyte suchas a polymer battery and the like.

The positive electrode of the battery of the present embodimentpreferably contains a transition metal compound containing a metal ionas the active material. More preferably, the positive electrode containsa lithium (Li)-containing transition metal compound as the activematerial. An example of the Li-containing transition metal compound isLiM₁-xM′xO₂, or LiM₂yM′O₄. Where, in the chemical formulae, 0≤x, y≤1,and M and M′ are respectively at least one kind of barium (Ba), cobalt(Co), nickel (Ni), manganese (Mn), chromium (Cr), titanium (Ti),vanadium (V), iron (Fe), zinc (Zn), aluminum (Al), indium (In), tin(Sn), scandium (Sc) and yttrium (Y).

However, the battery of the present embodiment may use other positiveelectrode materials such as transition metal chalcogenides; vanadiumoxide and lithium (Li) compound thereof; niobium oxide and lithiumcompound thereof; conjugated polymers using organic conductivesubstance; Shepureru phase compound; activated carbon; activated carbonfiber, and the like.

The electrolytic solution of the battery of the present embodiment isgenerally a nonaqueous electrolytic solution in which lithium salt asthe supporting electrolyte is dissolved into an organic solvent.Examples of lithium salt include LiClO₄, LiBF₄, LiPF₆, LiAsF₆,LiB(C₆H₅), LiCF₃SO₃, LiCH₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, Li(CF₂SO₂)₂,LiCl, LiBr, and Li. These may be used singly or in combination. Theorganic solvent is preferably carbonic ester, such as propylenecarbonate, ethylene carbonate, ethyl methyl carbonate, dimethylcarbonate, and diethyl carbonate. However, other various kinds oforganic solvents including carboxylate ester and ether are usable. Theseorganic solvents may be used singly or in combination.

The separator is placed between the positive electrode and the negativeelectrode. The separator serves as an insulator. Further, the separatorgreatly contributes to the retention of electrolyte. The battery of thepresent embodiment may include a well known separator. The separator ismade of, for example, polypropylene or polyethylene, which ispolyolefin-based material, or mixed fabric of the two, or a porous bodysuch as a glass filter. The above described negative electrode, positiveelectrode, separator, and electrolytic solution or electrolyte areaccommodated in a container to produce a battery.

Hereinafter, the negative electrode active material, the negativeelectrode, and the battery of the present embodiment described abovewill be described in more detail by using Examples. It is noted that thenegative electrode active material, the negative electrode, and thebattery of the present embodiment will not be limited to Examples shownbelow.

EXAMPLES

Powdered negative electrode active materials, negative electrodes, andcoin batteries of Inventive Examples 1 to 8 of the present invention andComparative Examples 1 to 4 shown in Table 1 were produced by thefollowing method. Then, changes in the crystal structure of eachnegative electrode active material caused by charging/discharging wereconfirmed. Further, discharge capacity (discharge capacity per volume)and cycle characteristics of each battery were investigated.

TABLE 1 Ceramics Ceramics content Battery characteristics with respectto total Initial Discharge Capacity amount of alloy Average dischargecapacity at retention Alloy phase chemical phase and ceramics grain sizecapacity 100th cycle ratio composition Composition (weight %) (μm)(mAh/cm³) (mAh/cm³) (%) Inventive Example 1 (1) Cu-23 at % Sn-5 at % SiAl₂O₃ 1 0.06 2985 2745 92.0 Inventive Example 2 (1) Cu-23 at % Sn-5 at %Si Al₂O₃ 5 0.06 2741 2534 92.4 Inventive Example 3 (1) Cu-23 at % Sn-5at % Si Al₂O₃ 45 0.06 1277 1123 87.9 Inventive Example 4 (1) Cu-23 at %Sn-5 at % Si FeSi 1 16.3 2941 2694 91.6 Inventive Example 5 (1) Cu-23 at% Sn-5 at % Si FeSi 5 16.3 2656 2434 91.6 Inventive Example 6 (1) Cu-23at % Sn-5 at % Si FeSi 45 16.3 1391 1354 97.3 Comparative Example 1 (1)Cu-23 at % Sn-5 at % Si Al₂O₃ 55 0.06 941 507 53.9 Comparative Example 2(1) Cu-23 at % Sn-5 at % Si — — — 1782 1502 84.3 Inventive Example 7 (2)Cu-20 at % Sn-10 at % Al Al₂O₃ 1 0.06 2977 2784 93.5 Comparative Example3 (2) Cu-20 at % Sn-10 at % Al — — — 1766 1455 82.4 Inventive Example 8(3) Cu-25 at % Sn Al₂O₃ 1 0.06 2865 2716 94.8 Comparative Example 4 (3)Cu-25 at % Sn — — — 1655 1313 79.3

Inventive Example 1 of the Present Invention

[Production of Negative Electrode Active Material]A mixture of aplurality of starting materials (elements) was high-frequency melted ina nozzle made of boron nitride in argon gas atmosphere to produce moltenmetal in such a way that the final chemical composition of the negativeelectrode active material excepting the ceramics part becomes thechemical composition listed in the “chemical composition” column inTable 1. A rapidly solidified foil band was produced by spraying themolten metal onto a rotating copper roll in the alloy phase. Thethickness of the foil band was 20 to 40 μm. This foil band waspulverized by a grinding machine (automatic mortar) into alloy powder ofa size of not more than 45 μm. This alloy powder was used as the alloymaterial before the ceramics was contained and dispersed therein. Thefinal chemical composition of the alloy phase in the negative electrodeactive material of Inventive Example 1 was as listed in the “chemicalcomposition” column in Table 1, and contained 23 at % of Sn and 5 at %of Si, with the balance being Cu.

The alloy material was blended with ceramics of the kind and the grainsize listed in Table 1 by the content listed in Table 1, and wassubjected to dispersion treatment by a high-speed planetary mill(product name “High G” manufactured by KURIMOTO, LTD.). In thisoccasion, the mass proportion (ball ratio) between the raw material(total of the alloy material and the ceramics) as the source of thenegative electrode active material and the ball was rawmaterial:ball=20:600. Moreover, the mill was operated for 12 hours in anargon inert gas atmosphere under a collision energy corresponding to 150G.

[Production of Negative Electrode]

The above powdered negative electrode active material, acetylene black(AB) as a conductive assistant, styrene-butadiene rubber (SBR) as abinder (2-fold dilution), and carboxymethylcellulose (CMC) as athickening agent were mixed in a mass ratio of 75:15:10:5 (blendingquantity was 1 g: 0.2 g: 0.134 g: 0.067 g). Then, a kneading machine wasused to produce a negative electrode mixture slurry by adding distilledwater to the mixture such that slurry density was 27.2%. Since thestyrene-butadiene rubber was used by being diluted 2-fold with water,0.134 g of styrene-butadiene rubber was blended when weighing.

The produced negative electrode mixture slurry was applied on a copperfoil by using an applicator (150 μm). The copper foil applied with theslurry was dried at 100° C. for 20 minutes. The copper foil after dryinghad a coating film made up of the negative electrode active material onthe surface. The copper foil having the coating film was subjected topunching to produce a disc-shaped copper foil having a diameter of 13mm. The copper foil after punching was pressed at a press pressure of500 kgf/cm² to produce a plate-shaped negative electrode material.

[Production of Battery]

The produced negative electrode, EC-DMC-EMC-VC-FEC as the electrolyticsolution, a polyolefin separator (ϕ17 mm) as the separator, and a metalLi plate (ϕ19×1 mmt) as the positive electrode material were prepared.Thus prepared negative electrode, the electrolytic solution, theseparator, and the positive electrode were used to produce a coinbattery of 2016 type. Assembly of the coin battery was performed withina glove box in argon atmosphere.

[Determination of Crystal Structure]

The crystal structures of the powdered negative electrode activematerial before using for the negative electrode, the negative electrodeactive material in the negative electrode before initial charging, andthe negative electrode active material in the negative electrode afterone to multiple times of charging and discharging were determined by thefollowing method. The concerned negative electrode active materials weresubjected to an X-ray diffraction measurement to obtain measured data.Then, based on the obtained measured data, crystal structures includedin the target negative electrode active materials were determined byRietveld method. More specifically, the crystal structures weredetermined by the following method.

(1) Crystal Structure Analysis of Powdered Negative Electrode ActiveMaterial Before Use in Negative Electrode

X-ray diffraction measurements were carried out for the powder of thenegative electrode active materials before use in the negative electrodeto obtain measured data of X-ray diffraction profile.

To be specific, SmartLab (product of Rigaku Co., Ltd) (rotor targetmaximum output 9 KW; 45 kV-200 mA) was used to obtain X-ray diffractionprofiles of the powder of the negative electrode active materials.

Based on the obtained X-ray diffraction profiles (measured data),crystal structures of alloy phases in the negative electrode activematerial were analyzed by Rietveld method.

The D0₃ ordered structure is an ordered structure as shown in FIG. 2A.In a Cu—Sn base alloy, mainly Cu is present at atomic sites shown by ablack circle and mainly Sn is present at atomic sites shown by a whitecircle, in FIG. 2A. Respective sites may be replaced by addition of athird element. It is known that such a crystal structure falls into No.225 (Fm-3m) of International Table (Volume-A) in the classification ofspace group representation. The lattice constant and atomic coordinatesof this space group number are as shown in Table 2.

TABLE 2 Parent phase (β₁ Phase), Crystal Structure: D0₃ Space GroupNumber (International Table A): No. 225 (Fm-3m) Lattice Constant: a =6.05 Å Atomic Multiplicity/ Atomic Coordinates Site Name Species WyckoffSymbol x y z Sn1 Sn 4a 0.0 0.0 0.0 Cu1 Cu 8c ¼ ¼ ¼ Cu2 Cu 4b ½ ½ ½

Accordingly, with the structure model of this space group number beingas the initial structure model of Rietveld analysis, a calculated valueof diffraction profile (hereinafter, referred to as a calculatedprofile) of β₁ phase (D0₃ structure) of this chemical composition wasfound by Rietveld method. RIETAN-FP (program name) was used for Rietveldanalysis.

Further, a calculated profile of the crystal structure of γ₁′ phase wasfound as well. The crystal structure of γ₁′ was 2H structure in thenotation of Ramsdell symbol, and the space group was No. 59-2 (Pmmn) ofInternational Table (Volume-A). The lattice constant and atomiccoordinates of No. 59-2 (Pmmn) are shown in Table 3.

TABLE 3 M Phase (γ₁′ Phase), Crystal Structure: 2H Space Group Number(International Table A): No. 59-2 (Pmmn) Lattice Constants: a = 4.379 Å,b = 5.498 Å, c = 4.615 Å Atomic Multiplicity/ Atomic Coordinates SiteName Species Wyckoff Symbol x y z Sn1 Sn 2b ¼ ¾ ⅙ Cu1 Cu 2a ¼ ¼ ⅙ Cu2 Cu4e ¼ 0.0 ⅔

A calculated profile was found by using RIETAN-FP supposing that thecrystal structure of the space group number of the above describe Table3 be the initial structure model of Rietveld analysis.

A result of the analysis revealed that a γ₁′ phase (2H structure) whichis a kind of M phase, and a β₁ phase (D0₃ tructure) which is the matrixphase of the γ₁′ phase (2H structure) were intermixed in the negativeelectrode active material of Inventive Example 1.

(2) Crystal Structure Analysis of Negative Electrode Active Material inNegative Electrode

The crystal structure of a negative electrode active material in anegative electrode before charging was also determined by the samemethod as that in (1). A measured X-ray diffraction profile was measuredby the following method.

The above described coin battery, which was before being charged, wasdisassembled within the glove box in argon atmosphere, and aplate-shaped negative electrode was taken out from the coin battery. Thenegative electrode taken out was enclosed in Myler foil (manufactured byDuPont). Thereafter, the perimeter of the Myler foil was sealed by athermocompression bonding machine. Then, the negative electrode sealedby the Myler foil was taken out of the glove box.

Next, a measurement sample was fabricated by bonding the negativeelectrode to a reflection-free sample plate manufactured by Rigaku Co.,Ltd. (a plate of a silicon single crystal which was cut out such that aspecific crystal plane was in parallel with the measurement plane) witha hair spray.

The measurement sample was mounted onto the X-ray diffraction apparatusdescribed below in (4), and the X-ray diffraction measurement of themeasurement sample was performed under measurement conditions describedbelow in (4).

(3) Analysis of Crystal Structure of Negative Electrode Active Materialin Negative Electrode after Charging and after Discharging

The crystal structure of the negative electrode active material in thenegative electrode after one to 100 times of charging and after one to100 times of discharging was also determined by the same method as thatin (1). The measured X-ray diffraction profile was measured by thefollowing method.

The above described coin battery was fully charged in acharging/discharging test apparatus. The fully charged coin battery wasdisassembled in the glove box, and a measurement sample was fabricatedby the same method as that in (2). The measurement sample was mountedonto the X-ray diffraction apparatus described below in (4), and X-raydiffraction measurement of the measurement sample was performed undermeasurement conditions described below in (4).

Moreover, the above described coin battery was fully discharged. Thefully discharged coin battery was disassembled in the glove box, and ameasurement sample was fabricated by the same method as in (3). Themeasurement sample was mounted onto the X-ray diffraction apparatusdescribed below in (4), and X-ray diffraction measurement of themeasurement sample was performed at measurement conditions describedbelow in (4).

For a negative electrode which had been subjected to charging anddischarging repeatedly in a coin battery, X-ray diffraction measurementwas performed by the same method.

(4) X-Ray Diffraction Apparatus and Measurement Conditions

-   -   Apparatus: SmartLab manufactured by Rigaku Co., Ltd.    -   X-ray tube: Cu-Kα ray    -   X-ray output: 45 kV, 200 mA    -   Incident monochrometer: Johannson-type crystal (which filters        out Cu-Kα₂ ray and Cu-Kβ ray)    -   Optical system: Bragg-Brentano geometry    -   Incident parallel slit: 5.0 degrees    -   Incident slit: ½ degree    -   Length limiting slit: 10.0 mm    -   Receiving slit 1: 8.0 mm    -   Receiving slit 2: 13.0 mm    -   Receiving parallel slit: 5.0 degrees    -   Goniometer: SmartLab goniometer    -   X-ray source—mirror distance: 90.0 mm    -   X-ray source—selection slit distance: 114.0 mm    -   X-ray source—sample distance: 300.0 mm    -   Sample—receiving slit 1 distance: 187.0 mm    -   Sample—receiving slit 2 distance: 300.0 mm    -   Receiving slit 1—receiving slit 2 distance: 113.0 mm    -   Sample—detector distance: 331.0 mm    -   Detector: D/Tex Ultra    -   Scan range: 10 to 120 degrees or 10 to 90 degrees    -   Scan step: 0.02 degrees    -   Scan mode: Continuous scan    -   Scanning speed: 2 degrees/min or 2.5 degrees/min

(5) Analysis Results of X-Ray Diffraction Measurement Data

(5-1)

From X-ray diffraction data obtained in (1), (2), and (3), it can beconfirmed that no significant reaction has occurred between the negativeelectrode active material and the electrolytic solution.

(5-2)

The X-ray diffraction profiles of “negative electrode active materialafter charging” and “negative electrode active material afterdischarging” revealed that the diffraction lines repeatedly changed in areversible manner at a position where the diffraction angle 20 is near37.5 to 39.0° (position caused by M phase (γ₁′ phase)) (hereinafter,referred to as a major diffraction line position). That is, structuralchanges were suggested.

(5-3)

Accordingly, the crystal structures of the “negative electrode activematerial after charging” and the “negative electrode active materialsafter discharging” were determined by using Rietveld method.

For example, in the negative electrode active material, the crystalplane A shown in FIG. 2D and the crystal plane B shown in FIG. 2C arealternately layered in the D0₃ structure of the matrix phase shown inFIGS. 1 and 2A. When a phase transformation occurs between the D0₃structure and γ₁′ phase which is a kind of M phase, as shown in FIGS. 2Aand 2B, the crystal plane B regularly undergoes shuffling due to shearstress, thereby being displaced to the position of crystal plane B′. Inthis case, phase transformation (M transformation) occurs withoutdiffusion of the host lattice. In the 2H structure after Mtransformation, the crystal plane A shown in FIG. 2D and the crystalplane B′ shown in FIG. 2E are alternately layered.

Then, it was judged whether the crystal structure of the negativeelectrode active material in the negative electrode of the presentexample involved M transformation or was not accompanied thereby (thatis, involved diffusion of host lattice at the time ofcharging/discharging) by comparing the measured data of the X-raydiffraction profiles of the negative electrode active material aftercharging and after discharging, the calculated profile of β₁ phase (D0₃structure), and the calculated profile of γ₁′ phase (2H structure).

In the X-ray diffraction profile, the intensity of the diffraction lineof near 37.5° to 39.9° increased as a result of initial charging, anddecreased as a result of consecutive discharging. It can be judged thatthis diffraction line resulted from the formation of M phase (γ₁′) by Mtransformation, as will be next described, from calculated profiles ofRIETAN-FP).

To be specific, in the 2H structure, an intensity peak appeared at 37.5to 39.0° of the X-ray diffraction profile. On the other hand, in the D0₃structure, no intensity peak appeared at 37.5 to 39.0°. In contrast tothis, in the X-ray diffraction profile after charging, an intensity peakappeared at 37.5 to 39.0°. On the other hand, in the X-ray diffractionprofile after discharging, no intensity peak appeared at 37.5 to 39.0°.Further, the intensity peak at 37.5 to 39.0° did not appear in the X-rayprofile (simulation result) of any crystal structure other than 2H.Here, the reason why the angle range of the diffraction line is as wideas 37.5 to 39.0° as stated above was that due to the effect of themechanical grinding (MG) in the ceramics dispersion process, strain wasintroduced in the material, thus resulting in increase in the half valuewidth.

From the above, the negative electrode of the present Example containedan alloy phase which underwent M transformation to become M phase (2Hstructure) as a result of charging, and became a matrix phase (D0₃structure) as a result of discharging. That is, the negative electrodeof the present Example contained an alloy phase which underwent Mtransformation when occluding lithium ions which are metal ions, andunderwent reverse transformation when releasing lithium ions.

In the negative electrode of the present Example, M transformation atthe time of charging, and reverse transformation at the time ofdischarging were repeated.

[Charge-Discharge Performance Evaluation of Coin Battery]

Next, discharge capacity and cycle characteristics of the battery ofInventive Example 1 were evaluated.

Constant current doping (corresponding to the insertion of lithium ionsinto electrode, and the charging of lithium ion secondary battery) wasperformed to a coin battery at a current value of 0.1 mA (a currentvalue of 0.075 mA/cm²) or a current value of 1.0 mA (a current value of0.75 mA/cm²) until the potential difference against the counterelectrode becomes 0.005 V. Thereafter, doping capacity was measured bycontinuing doping against the counter electrode at a constant voltageuntil the current value became 7.5 μA/cm² while retaining 0.005 V.

Next, de-doping capacity was measured by performing de-doping (whichcorresponds to desorption of lithium ions from the electrode, anddischarge of the lithium ion secondary battery) at a current value of0.1 mA (a current value of 0.075 mA/cm²) or a current value of 1.0 mA (acurrent value of 0.75 mA/cm²) until the potential difference becomes 1.2V.

The doping capacity and de-doping capacity correspond to charge capacityand discharge capacity when the electrode is used as the negativeelectrode of the lithium ion secondary battery. Therefore, the measureddope capacity was defined as the charge capacity, and a measuredde-doping capacity was defined as the discharge capacity.

Charging and discharging were repeated 100 times at the same conditionsas described above. Then, a capacity retention ratio (%) was defined as“the discharge capacity at the time of de-doping of the 100th cycle”divided by “the discharge capacity at the time of de-doping of the 1stcycle, and multiplied by 100. In the coin battery of Inventive Example1, the initial discharge capacity, the discharge capacity of the 100thcycle, and the capacity retention ratio were as listed in Table 1.

Inventive Examples 2 and 3

In Inventive Examples 2 and 3, negative electrode active materials,negative electrodes, and coin batteries were produced in the same way asin Inventive Example 1 excepting that the ceramics contents were changedas shown in Table 1.

Determination of crystal structure, and evaluation of oxygenconcentration in the negative electrode active material and variouscharge-discharge performances of the coin battery were performed in thesame way as in Inventive Example 1.

The result of the determination of crystal structure was the same as inInventive Example 1. That is, it was confirmed that the alloy phases ofInventive Examples 2 and 3 had a crystal structure that undergoes Mtransformation when occluding lithium ions, and undergoes reversetransformation when releasing lithium ions. The results of theevaluation of various charge-discharge performances of the coin batterywere as shown in Table 1.

Inventive Examples 4 to 6

In Inventive Examples 4 to 6, negative electrode active materials,negative electrodes, and coin batteries were produced in the same way asin Inventive Example 1 excepting that the kinds, the contents, and theaverage grain sizes before being charged into an attritor mill of theceramics were changed as shown in Table 1.

Determination of crystal structure, and evaluation of variouscharge-discharge performances of the coin battery were performed in thesame way as in Inventive Example 1. The result of the determination ofcrystal structure was the same as in Inventive Example 1. That is, itwas confirmed that the alloy phases of Inventive Examples 4 to 6 had acrystal structure that undergoes M transformation when occluding lithiumions, and undergoes reverse transformation when releasing lithium ions.Results of the evaluation of various charge-discharge performances ofthe coin battery were as shown in Table 1.

Comparative Examples 1 and 2

In Comparative Example 1, the negative electrode active material,negative electrode, and coin battery were produced in the same way as inInventive Example 1 excepting that the content of ceramics was changedas shown Table 1. In Comparative Example 2, the negative electrode andcoin battery were produced in the same way as in Inventive Example 1excepting that the alloy material obtained by pulverization with thegrinding machine (automatic mortar) in Inventive Example 1 was used asthe negative electrode active material. The negative electrode activematerial of Comparative Example 2 did not contain ceramics.

Determination of crystal structure, and evaluation of variouscharge-discharge performances of the coin battery were performed in thesame way as in Inventive Example 1. The result of the determination ofcrystal structure was the same as in Inventive Example 1. That is, itwas confirmed that the alloy phases of Comparative Examples 1 and 2 hada crystal structure that undergoes M transformation when occludinglithium ions, and undergoes reverse transformation when releasinglithium ions. The results of evaluation of various charge-dischargeperformances of the coin battery were as shown in Table 1.

Inventive Example 7 and Comparative Example 3

In Inventive Example 7, the negative electrode active material, negativeelectrode, and coin battery were produced in the same way as inInventive Example 1 excepting that the final chemical composition of thealloy phase was changed to the composition according to Table 1.

In Comparative Example 3, the negative electrode and coin battery wereproduced in the same way as in Inventive Example 7 excepting that thealloy material obtained by pulverization with the grinding machine(automatic mortar) in Inventive Example 7 was used as the negativeelectrode active material. The negative electrode active material ofComparative Example 3 did not contain any ceramics.

Determination of crystal structure, and evaluation of variouscharge-discharge performances of the coin battery were performed in thesame way as in Inventive Example 1. The result of the determination ofcrystal structure was the same as in Inventive Example 1. That is, itwas confirmed that the alloy phases of Inventive Examples 7 andComparative Example 3 had a crystal structure that undergoes Mtransformation when occluding lithium ions, and undergoes reversetransformation when releasing lithium ions. The results of evaluation ofvarious charge-discharge performances of the coin battery were as shownin Table 1.

Inventive Example 8 and Comparative Example 4

In Inventive Example 8, the negative electrode active material, negativeelectrode, and coin battery were produced in the same way as inInventive Example 1 excepting that the final chemical composition of thealloy phase was changed to the composition according to Table 1.

In Comparative Example 4, the negative electrode and coin battery wereproduced in the same way as in Inventive Example 8 excepting that thealloy material obtained by pulverization with the grinding machine(automatic mortar) in Inventive Example 8 was used as the negativeelectrode active material. The negative electrode active material ofComparative Example 4 did not contain any ceramics.

Determination of crystal structure, and evaluation of variouscharge-discharge performances of the coin battery were performed in thesame way as in Inventive Example 1. According to the determination ofcrystal structure, the alloy phase changed as follows ascharging/discharging was performed. The alloy phase was M phase (2Hstructure) before initial charging, M phase (2H structure) after initialcharging, and a matrix phase (D0₃ structure) after initial discharging,and thereafter, the alloy phase underwent M transformation to betransformed into M phase (2H structure) through charging, and becamematrix phase (D0₃ structure) through discharging. That is, it wasconfirmed that the negative electrodes of Inventive Examples 8 andComparative Example 4 had an alloy phase that undergoes M transformationwhen occluding lithium ions which are metal ions, and undergoes reversetransformation when releasing lithium ions. Results of evaluatingvarious charge-discharge performances of the coin battery were as shownin Table 1.

With reference to Table 1, in each of Inventive Examples 1 to 8, thecontent of ceramics with respect to the total mass of the alloy phase ofthe ceramics was more than 0 to 50 mass %. As a result of that, thedischarge capacities at the initial and the 100th cycles were as high asnot less than 1100 mAh/cm³ and the capacity retention ratio was not lessthan 85% as well.

On the other hand, in Comparative Example 1, the ceramics content wasmore than 50 mass %. As a result of that, the discharge capacity waslow, and the capacity retention ratio was less than 90% as well. InComparative Examples 2 to 4, the ceramics was not contained. As a resultof that the capacity retention ratio was less than 90%.

So far, embodiments of the present invention have been described.However, the above described embodiments are merely examples to carryout the present invention. Therefore, the present invention will not belimited to the above described embodiments, and can be carried out byappropriately modifying the above described embodiments within a rangenot departing from the spirit thereof.

The invention claimed is:
 1. A negative electrode active material,comprising: an alloy phase which undergoes thermoelastic diffusionlesstransformation when releasing metal ions or occluding the metal ions;and ceramics included in and dispersed in the alloy phase, wherein thecontent of the ceramics in the alloy phase is more than 0 to 50 mass %with respect to a total mass of the alloy phase and the ceramics.
 2. Thenegative electrode active material according to claim 1, wherein theceramics contains one or more kinds selected from the group consistingof Al₂O₃, FeSi, SiC, Si₃N₄, TiC, TiB₂, Y₂O₃, ZrB₂, HfB₂, ZrO₂, ZnO, WC,W₂C, CrB₂, BN, and CeO₂.
 3. The negative electrode active materialaccording to claim 2, wherein the alloy phase undergoes thermoelasticdiffusionless transformation when occluding the metal ions, andundergoes reverse transformation when releasing the metal ions.
 4. Thenegative electrode active material according to claim 3, wherein thealloy phase after the thermoelastic diffusionless transformationcontains a crystal structure which is 2H in Ramsdell notation, and thealloy phase after the reverse transformation contains a crystalstructure which is D0₃ in Strukturbericht notation.
 5. The negativeelectrode active material according to claim 4, wherein the alloy phasecontains Cu and Sn.
 6. The negative electrode active material accordingto claim 5, wherein the alloy phase contains 10 to 20 at % or 21 to 27at % of Sn, with the balance being Cu and impurities.
 7. The negativeelectrode active material according to claim 5, wherein the alloy phasefurther contains, in place of a part of Cu, one or more kinds selectedfrom the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B,and C.
 8. The negative electrode active material according to claim 7,wherein the alloy phase contains: Sn: 10 to 35 at %; and one or morekinds selected from the group consisting of Ti: 9.0 at % or less, V:49.0 at % or less, Cr: 49.0 at % or less, Mn: 9.0 at % or less, Fe: 49.0at % or less, Co: 49.0 at % or less, Ni: 9.0 at % or less, Zn: 29.0 at %or less, Al: 49.0 at % or less, Si: 49.0 at % or less, B: 5.0 at % orless, and C: 5.0 at % or less, with the balance being Cu and impurities.9. The negative electrode active material according to claim 3, whereinthe alloy phase contains Cu and Sn.
 10. The negative electrode activematerial according to claim 9, wherein the alloy phase contains 10 to 20at % or 21 to 27 at % of Sn, with the balance being Cu and impurities.11. The negative electrode active material according to claim 9, whereinthe alloy phase further contains, in place of a part of Cu, one or morekinds selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni,Zn, Al, Si, B, and C.
 12. The negative electrode active materialaccording to claim 11, wherein the alloy phase contains: Sn: 10 to 35 at%; and one or more kinds selected from the group consisting of Ti: 9.0at % or less, V: 49.0 at % or less, Cr: 49.0 at % or less, Mn: 9.0 at %or less, Fe: 49.0 at % or less, Co: 49.0 at % or less, Ni: 9.0 at % orless, Zn: 29.0 at % or less, Al: 49.0 at % or less, Si: 49.0 at % orless, B: 5.0 at % or less, and C: 5.0 at % or less, with the balancebeing Cu and impurities.
 13. The negative electrode active materialaccording to claim 1, wherein the alloy phase undergoes thermoelasticdiffusionless transformation when occluding the metal ions, andundergoes reverse transformation when releasing the metal ions.
 14. Thenegative electrode active material according to claim 13, wherein thealloy phase after the thermoelastic diffusionless transformationcontains a crystal structure which is 2H in Ramsdell notation, and thealloy phase after the reverse transformation contains a crystalstructure which is D0₃ in Strukturbericht notation.
 15. The negativeelectrode active material according to claim 14, wherein the alloy phasecontains Cu and Sn.
 16. The negative electrode active material accordingto claim 15, wherein the alloy phase contains 10 to 20 at % or 21 to 27at % of Sn, with the balance being Cu and impurities.
 17. The negativeelectrode active material according to claim 15, wherein the alloy phasefurther contains, in place of a part of Cu, one or more kinds selectedfrom the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B,and C.
 18. The negative electrode active material according to claim 17,wherein the alloy phase contains: Sn: 10 to 35 at %; and one or morekinds selected from the group consisting of Ti: 9.0 at % or less, V:49.0 at % or less, Cr: 49.0 at % or less, Mn: 9.0 at % or less, Fe: 49.0at % or less, Co: 49.0 at % or less, Ni: 9.0 at % or less, Zn: 29.0 at %or less, Al: 49.0 at % or less, Si: 49.0 at % or less, B: 5.0 at % orless, and C: 5.0 at % or less, with the balance being Cu and impurities.19. The negative electrode active material according to claim 13,wherein the alloy phase contains Cu and Sn.
 20. The negative electrodeactive material according to claim 19, wherein the alloy phase contains10 to 20 at % or 21 to 27 at % of Sn, with the balance being Cu andimpurities.
 21. The negative electrode active material according toclaim 19, wherein the alloy phase further contains, in place of a partof Cu, one or more kinds selected from the group consisting of Ti, V,Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B, and C.
 22. The negative electrodeactive material according to claim 21, wherein the alloy phase contains:Sn: 10 to 35 at %; and one or more kinds selected from the groupconsisting of Ti: 9.0 at % or less, V: 49.0 at % or less, Cr: 49.0 at %or less, Mn: 9.0 at % or less, Fe: 49.0 at % or less, Co: 49.0 at % orless, Ni: 9.0 at % or less, Zn: 29.0 at % or less, Al: 49.0 at % orless, Si: 49.0 at % or less, B: 5.0 at % or less, and C: 5.0 at % orless, with the balance being Cu and impurities.
 23. A negativeelectrode, comprising the negative electrode active material accordingto claim
 1. 24. A battery, comprising the negative electrode accordingto claim 23.